Question

等比数列an中,s5=31,s6-a1=62,an=

Answer 1

1.

S5=a1(1-q^5)/(1-q)=31

S6-a1=a1(1-q^6)/(1-q)-a1=62

联立求得：

a1=1;q=2

an=2^(n-1)

2.

an=1+2+2^2+……+2^(n-1)=2^n-1

Sn=a1+a2+a3+……+an

＝(2^1+2^2+2^3+……+2^n)-n

＝2^(n+1)-n